Fix all Svelte 5 const violations in components

Changed const to let for all $state and $bindable destructuring:
- RetroInput.svelte: $bindable props
- ModernJourneyFull.svelte: showSlackPreview, timelinePosition
- ModernJourneyContent.svelte: showComparison
- DebugRetro.svelte: sessions
- app-info/+page.svelte: failedImages

Tests now load properly (500 error fixed), failing on Appium being off as expected.

Author: nirukk52

frontend/src/lib/components/DebugRetro.svelte

@@ -20,7 +20,7 @@ interface DebugSession {
 /**
  * Debugging sessions we've done
  */
-const sessions: DebugSession[] = $state([
+let sessions: DebugSession[] = $state([
   {
     id: "graph-stream-connection",
     title: "Graph Stream WebSocket Connection Failure",

frontend/src/lib/components/ModernJourneyContent.svelte

@@ -33,7 +33,7 @@ import {
 let activeStep = $state(0);
 
 /** Show comparison view on impact page */
-const showComparison = $state(false);
+let showComparison = $state(false);
 
 /** Journey steps configuration */
 const steps = [

frontend/src/lib/components/ModernJourneyFull.svelte

@@ -33,10 +33,10 @@ import {
 } from "lucide-svelte";
 
 /** Show Slack notification preview */
-const showSlackPreview = $state(false);
+let showSlackPreview = $state(false);
 
 /** Timeline scrubber position (0-10) */
-const timelinePosition = $state(0);
+let timelinePosition = $state(0);
 
 /** Calculated confidence based on timeline */
 const confidence = $derived(75 + timelinePosition * 2);